HVAC Load Estimation

7/29/2019 HVAC Load Estimation

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MEBS6006 Environmental Services Ihttp://www.hku.hk/bse/MEBS6006/

Load Estimation

Dr. Sam C M Hui

The University of Hong Kong

- .

Sep 2009


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Contents

Basic Conce ts

Outdoor Design Conditions

Indoor Design Conditions

Coolin Load Com onents

Cooling Load Principles

Cooling Coil Load

Software Applications


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Basic Concepts

Conduction

Convection

Thermal properties of building materials

Overall thermal transmittance (U-value)

Thermal capacity (specific heat)Q = U A (t)


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Four forms of heat transfer

CONVECTION

(Source: Food and Agriculture Organization of the United Nations, www.fao.org)


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Basic Concepts

level)

Sensible heat transfer rate:

= 1.23 Flow rate L/s t

Latent heat transfer rate:

qlatent

= ow ra e, s w

Total heat transfer rate:

qtotal = 1.2 (Flow rate, L/s) (h)

total sensible latent


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Basic Concepts

The amount of heat that must be added or removed

rom t e space to ma nta n t e proper temperature

in the space When thermal loads push conditions outside

o e com or range, sys ems are useto bring the thermal conditions back to

comfort conditions


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Basic Concepts

Calculate peak design loads (cooling/heating)

Estimate likely plant/equipment capacity or size

. .

Form the basis for building energy analysis

Cooling load is our main target

Affect building performance & its first cost


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Basic Concepts

General rocedure for coolin load calculations

Obtain the characteristics of the building, building

, , .

specifications

, ,

shading (like adjacent buildings)

a n appropr a e wea er a a an se ec ou oor es gn

conditions

e ec n oor es gn con ons nc u e perm ss e

variations and control limits)


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Basic Concepts

General rocedure for coolin load calculations

(contd)

, ,

equipment appliances and processes that would contribute

Select the time of day and month for the cooling load

Calculate the space cooling load at design conditions

Assess the cooling loads at several different time or a

design day to find out the peak design load


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Cooling load profiles


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Basic Concepts

A buildin surve will hel us achieve arealistic estimate of thermal loads

Use of spaces Physical dimensions of spaces

Columns and beams

onstruct on mater a s

Surrounding conditions Windows, doors, stairways


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(Source: ASHRAE Handbook Fundamentals 2005)


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Basic Concepts

People (number or density, duration of occupancy,

nature o act v ty

Li htin W/m2 t e Appliances (wattage, location, usage)

Vent at on cr ter a, requ rements Thermal stora e if an

Continuous or intermittent operation


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Basic Concepts

1. Rough estimates of design loads & energy use

Such as by rules of thumb & floor areas

See Cooling Load Check Figures See references for some examples of databooks

. ,

building info, system info)

Building layouts & plans are developed

3. Perform detailed load & ener calculations


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OutdoorDesign ConditionsOutdoorDesign Conditions

Climatic design information

General info: e.g. latitude, longitude, altitude,

Outdoor design conditions include

Derived from statistical analysis of weather data Typical data can be found in handbooks/databooks,

such as ASHRAE Fundamentals Handbook


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Climatic desi n info from ASHRAE

Previous data & method (before 1997)

, ,

Based on 1%, 2.5% & 5% nos. hours of occurrence

ew met o un amenta s + :

Based on annual percentiles and cumulative frequencyo occurrence, e.g. 0.4%, 1%, 2% (o w o e year)

More info on coincident conditions

Findings obtained from ASHRAE research projects Data can be found on a relevant CD-ROM


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,

Annual heating & humidif. design conditions

Coldest month

Heating dry-bulb (DB) temp. Humidification dew point (DP)/ mean coincident dry-

bulb tem . MCDB and humidit ratio HR

Coldest month wind speed (WS)/mean coincident dry-

Mean coincident wind speed (MCWS) & prevailing

.

(Latest information from ASHRAE Handbook Fundamentals 2009)


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,

Cooling and dehumidification design conditions

Hottest month and DB range

Cooling DB/MCWB: Dry-bulb temp. (DB) + Meancoincident wet-bulb temp. (MCWB)

Eva oration WB/MCDB: Web-bulb tem . WB +

Mean coincident dry-bulb temp. (MCDB)

Dehumidification DP/MCDB and HR: Dew-point temp.

Enthalpy/MCDB


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,

Extreme annual design conditions

Monthly climatic design conditions

Tem erature de ree-da s and de ree-hours

Monthly design DB and mean coincident WB

Mean daily temperature range

Clear sky solar irradiance


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Joint frequency tables of psychrometric conditions

Annual, monthly and hourly data

De ree-da s coolin /heatin & climatic normals To classify climate characteristics

yp ca year a a se s year: , ours

For energy calculations & analysis

Recommended Outdoor Design Conditions for Hong Kong


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Recommended Outdoor Design Conditions for Hong Kong

ocation ong ong atitu e , ongitu e , e evation m

Weather station Royal Observatory Hong Kong

Summer months June to September (four hottest months), total 2928 hours

Winter months December, January & February (three coldest months), total 2160 hours

Desi n For comfort HVAC based on For critical rocesses based ontemperatures: summer 2.5% or annualised 1% and

winter 97.5% or annualised 99.3%)summer 1% or annualised 0.4% and

winter 99% or annualised 99.6%)

Summer Winter Summer Winter

DDB / CWB 32.0 oC / 26.9 oC 9.5 oC / 6.7 oC 32.6 oC / 27.0 oC 8.2 oC / 6.0 oC

CDB / DWB 31.0 oC / 27.5 oC 10.4 oC / 6.2 oC 31.3 oC / 27.8 oC 9.1 oC / 5.0 oC

Note: 1. DDB is the design dry-bulb and CWB is the coincident wet-bulb temperature withit; DWB is the design wet-bulb and CDB is the coincident dry-bulb with it.

2. The design temperatures and daily ranges were determined based on hourly data

(Source: Research findings from Dr. Sam C M Hui)

-based on extreme values between 1884-1939 and 1947-1994.

Recommended Outdoor Design Conditions for Hong Kong (contd)


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Recommended Outdoor Design Conditions for Hong Kong (cont d)

xtremetemperatures:

ottest mont : u y

mean DBT = 28.6o

C

o est mont : anuary

mean DBT = 15.7o

Ca so ute max. = . o

mean daily max. DBT = 25.7 oC

a so ute min. = . o

mean daily min. DBT = 20.9 oC

Diurnal range: Summer Winter Whole year- Mean DBT 28.2 16.4 22.8

- Daily range 4.95 5.01 5.0

Wind data: Summer Winter Whole year- Wind direction 090 (East) 070 (N 70 E) 080 (N 80 E)

- Wind speed 5.7 m/s 6.8 m/s 6.3 m/s

Note: 3. Wind data are the prevailing wind data based on the weather summary for the 30-year period 1960-1990. Wind direction is the prevailing wind direction in degreesclockwise from north and the wind speed is the mean prevailing wind speed.

(Source: Research findings from Dr. Sam C M Hui)


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IndoorDesign ConditionsIndoorDesign Conditions

Basic desi n arameters: for thermal comfort

Air temp. & air movement

- -

Air velocity: summer < 0.25 m/s; winter < 0.15 m/s

e at ve um ty

Summer: 40-50% (preferred), 30-65 (tolerable)

Winter: 25-30% (with humidifier); not specified (w/ohumidifier)

See also ASHRAE Standard 55-2004

ASHRAE comfort zone

AS A C f


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ASHRAE Comfort Zones

ase on vers on o tan ar


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IndoorDesign ConditionsIndoorDesign Conditions

Indoor air ualit : for health & well-bein

Air contaminants

. . , , ,

Outdoor ventilation rate provided ASHRAE Standard 62-2007

Air cleanliness (e.g. for processing), air movement

Other design parameters: oun eve no se cr er a

Pressure differential between the space &surroundings (e.g. +ve to prevent infiltration)


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(NC = noise critera; RC = room criteria)


* Remark: buildings in HK often have higher NC, say add 5-10 dB (more noisy).


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Cooling Load Components

1. Heat gain through exterior walls and roofs

2. Solar heat gain through fenestrations (windows)

.

4. Heat gain through partitions & interior doors

Internal.

2. Electric lights

3. Equipment and appliances


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Air leakage and moisture migration, e.g.

flow of outdoor air into a building through

, ,of exterior doors for entrance

System (HVAC) Outdoor ventilation air

,reheat, fan & pump energy, energy recovery


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Sensible cooling load + Latent cooling load

= (sensible items) + (latent items)

c components ave atent oa s c

onl have sensible load? Wh ?

Three major parts for load calculation

External cooling load

Ventilation and infiltration air


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Example: CLTD/SCL/CLF method

It is a one-step, simple calculation procedure developed

by ASHRAE CLTD = cooling load temperature difference

SCL = solar coolin load

CLF = cooling load factor

Tables for CLTD, SCL and CLF


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Roofs, walls, and glass conduction

q = U (CLTD) U= U-value;A = area

Solar load throu h lass q =A (SC) (SCL) SC = shading coefficient

Partitions, ceilings, floors q = U (tadjacent - tinside)


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People

qsensible =N(Sensible heat gain) (CLF)

qlatent =N(Latent heat gain) Lights

ul sa Ful = lighting use factor; Fsa = special allowance factor

pp ances

qsensible = qin ut x usage factors (CLF)

qlatent = qinput x load factor (CLF)


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qsensible = 1.23 Q (toutside - tinside)

qlatent = 3010 Q (woutside - winside)

= -tota . outs e ns e

System heat gain

Fan heat gain

Ceiling return air plenum


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Space a volume w/o a partition, or a partitioned

room, or group o rooms

Room an enclosed s ace a sin le load Zone a space, or several rooms, or units of space

v

operating characteristics

Thermal zoning


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Definitions

Space heat gain: instantaneous rate of heat gain

Space cooling load: the rate at which heat must beremove rom e space o ma n a n a cons anspace air temperature

Space heat extraction rate: the actual rate of heatremoval when the space air temp. may swing

Cooling coil load: the rate at which energy is

removed at a coolin coil servin the s ace


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Conversion of heat gain into cooling load



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Instantaneous heat ain vs s ace coolin loads

They are NOT the same

ect o eat storage

Ni ht shutdown eriod HVAC is switched off. What happens to the space?

- -

When HVAC system begins to operate

Conditioning period

Space air temperature within the limits


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S ace load and e ui ment load

Space heat gain (sensible, latent, total)

pace coo ng ea ng oa a u ng

Space heat extraction rate Cooling / heating coil load [at air-side system]

Instantaneous heat gain Convective heat


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Convective and radiative heat in a conditioned space

(Source: Wang, S. K., 2001.Handbook of Air Conditioning and Refrigeration, 2nd ed.)


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Shows the variation of space cooling load

Such as 24-hr cycle

What factors will affect load profiles?

Peak load and block load.

Block load = sum of zone loads at a specific time

Cooling load profiles

ota coo ng oa


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ota coo ng oa

(Source: D.G. Stephenson, 1968)

North


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South

Block load and thermal zoning

Cooling loads due to windows at different orientations


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(Source: D.G. Stephenson, 1968)

Profiles of solar heat gain (July) (for latitude 48 deg N)


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(Source: Keith E. Elder)

Solar cooling load vs. heat gain (July, west) (latitude 48 deg N)


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(Source: Keith E. Elder)

C li L d P i i l


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Two paths:

Moisture migrates in building envelope

Air leakage (infiltration or exfiltration) If slight RH variation is acceptable, then storage

Latent heat gain = latent cooling load (instantaneously)

What happens if both temp. & RH need to be

Cooling Coil Load


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Cooling Coil Load

Space cooling load (sensible & latent)

Supply system heat gain (fan + air duct)

Load due to outdoor ventilation rates (or

vent at on oa

conditioning cycle on a psychrometric chart?

See also notes in Psychrometrics

Typical summer air conditioning cycle


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Cooling coil load ent at on oa

Return system heat gain

Space cooling load


Cooling Coil Load


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Cooling Coil Load

(kW)loadSensible

(L/s)airflowSupply

To determine supply air flow rate & size of air

system, ucts, term na s, users

It is a com onent of coolin coil load Infiltration heat gain is an instant. cooling load

Cooling coil load

refrigeration system

Remem er, vent at on oa s a co oa

Heating Load


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Heating Load

Max. heat energy required to maintain winter

n oor es gn temp.

Usually occurs before sunrise on the coldest days

Include transmission losses & infiltration/ventilation

All heating losses are instantaneous heating loads Credit for solar & internal heat gains is not included

Latent heat often not considered (unless w/ humidifier)

Thermal storage effect of building structure is ignored

Heating Load


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Heating Load

-

conditions based on Design interior and exterior conditions

No solar effect (at night or on cloudy winter days)

Before the presence of people, light, and

a liances has an offsettin effect

Also, a warm-up/safety allowance of 20-25%

s a r y common


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TRACE 600/700 and Carrier E20-II

Commercial programs from Trane and Carrier

Most widely used by engineers DOE-2 (used more for research)



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TRACE 700

TRACE = Trane Air Conditioning Economics

Demon version can be downloaded

p: www. rane.com commerc a

References


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References

Air Conditionin and Refri eration En ineerin

(Wang and Norton, 2000)

ASHRAE Handbook Fundamentals (2009 edition)

Chapter 14 Climatic Design Information

Cha ter 15 Fenestration

Chapter 17 Residential Cooling and Heating Load

Chapter 18 Nonresidential Cooling and Heating Load

References


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References

Load & Energy Calculations inASHRAE

an oo un amenta s

The followin revious coolin load calculationsare described in earlier editions of the ASHRAE

CLTD/SCL/CLF method met o

TFM method

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HVAC Load Estimation

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